I. Field of the Invention
This invention relates generally to apparatus for cutting through webs of thin, flexible thermoplastic sheet material and more specifically to an improved cutting arrangement for use in a machine for fabricating plastic bags and the like.
II. Discussion of the Prior Art
In plastic bag making machinery and in other packaging machinery, a need exists for cutting apparatus which may be brought into play to sever superposed webs of thermoplastic sheet material into discrete segments while at the same time creating a fused weld between the superposed sheets along the length of the cut. Many such machines have used a relatively heavy heated blade which is brought into contact with the thermoplastic sheet material in such a fashion that the material is compressed causing a nearly complete separation of the web. A secondary step follows to pull the web apart, thus leaving an edge seal on the trailing edge of the separated part and a similar seal on the leading edge of the webs. These blades tend to be relatively heavy requiring considerable force to accelerate and decelerate them during their cutting stroke. This generally puts a considerable strain on the machine parts employed to move the massive blade in a reciprocating fashion relative to the web of sheet material being cut. It also requires an inordinate amount of time to manipulate the blade between successive strokes which reduces the production rate of the machine.
One prior art approach which has been used to obviate certain of the problems associated with reciprocally moving a massive blade is the use of a hot-wire type cutting implement. Such a hot-wire is generally resistively heated by passing a predetermined current through the wire. Being made from relatively fine wire, they are light in weight and are more readily manipulated in their cutting stroke because of their substantially lower inertia. However, because such hot-wires are fabricated from relatively fine wire, their heat capacity tends to be quite low. As such, when the hot-wire is forced against the thermoplastic sheet material to be cut, there is a tendency of the mass of plastic material to draw heat from the wire causing it to cool. Hence, the next cutting stroke must be delayed until the wire again reaches a predetermined temperature suitable for the cutting operation. The delay introduced between each cutting stroke necessarily reduces the throughput of the machine on which it is used.
Where cooling of the hot-wire is a limiting factor on the rate at which cuts may be made, it might appear that the obvious remedy would be to increase the current in the current flow and therefore the temperature of the wire. However, as those skilled in the packaging and bag making machinery art know, there is a practical limit to the temperature which may be used in cutting thermoplastic sheet material with a hot-wire, especially where it is desired that the portion of the film being cut through create a fusion bond with a superposed sheet. If the hot-wire temperature is too high, the thermoplastic material will effectively vaporize and will fail to create a fused seam or seal. Furthermore, the cutting wire will have a relatively short life when made to operate at inordinately high temperatures.
The present invention obviates the problems of prior art hot-wire cutting apparatus used in bag making machinery and related equipment. In accordance with the present invention, the hot-wire is made to progressively move through the superposed webs to be cut from one side edge thereof to the other in such a fashion that the point of contact between the hot-wire and the surface being cut moves across the length of the hot-wire during the cutting stroke. Furthermore, the opposed ends of the hot-wire are manipulated such that on a return cutting stroke following the indexing of the web material, the portion of the hot-wire which first came in contact with the web during the preceding cutting stroke again is the first to contact the web during the next cutting stroke. As a result, the points of the wire again begin to re-heat following their contact with the web material, and by the time that the first cut has been completed and the web of thermoplastic sheet material has been stepped to its next position, the point of the wire first to contact the web will again have reached its appropriate temperature for ensuring simultaneous cutting and sealing.